The first is a bit more mature and is used widely in the field. The second is a recent development, but is coming from 10gen, the creators of mongodb. Implementation is looking good and is rather like the samus driver. If you need in production something right now, I'm not sure what to advise, but in the long run, I'd go for the 10gen driver.

The one thing that it currently doesn't offer is Linq integration. This could be important to you.

I'm actually using the first right now and I found It pretty cool . but why u advise me to use the 10gen driver instead ?!
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Rawhi Jan 24 '11 at 9:52

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@rahwi:I'm not advising that. I expect that the 10gen driver will be a more future proof choice if you start now. The company that brought us mongodb is backing this project as the driver for the .NET platform. The two are very comparable in design. Switching wouldn't be hard, but I'd definitely not jump now.
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Teun DJan 25 '11 at 21:44